Field of the Invention
The present invention relates to a roller, especially a pressure roller provided in a fixing device in image forming apparatus such as electrophotographic copying machines and electrophotographic printers, and to a fixing device including the roller.
Description of the Related Art
In related-art image forming apparatus, for example, a film heating system has been widely used in a fixing device configured to fix an unfixed image (toner image), which is formed and borne on a recording material, onto a surface of the recording material under heating.
The fixing device includes a heater serving as a heating member, a fixing film serving as a flexible rotary member configured to rotate while being heated in contact with the heater, and a pressure roller serving as a pressure member forming a fixing nip portion with the heater through intermediation of the fixing film. A recording material having an unfixed toner image borne thereon is introduced between the fixing film and the pressure roller in the fixing nip portion, and is nipped and conveyed simultaneously with the fixing film to fix the unfixed toner image onto a surface of the recording material while heat is applied from the heater through the fixing film and pressure is applied at the fixing nip portion.
However, in such a related-art heating fixing device adopting the film heating system, when a sheet being a recording material passes through the fixing device, wrinkles may be formed on the sheet at the fixing nip portion due to temperature unevenness in a longitudinal direction of the pressure roller and unevenness of a sheet conveyance speed at the fixing nip portion in the longitudinal direction of the pressure roller.
The structure in which a radially outer portion of a pressure roller has a reverse camber shape as disclosed in Japanese Patent Application Laid-Open No. H04-44076 is known to deal with the wrinkles of the recording material. With this structure, a circumferential speed of the pressure roller becomes higher at both end portions than in a middle portion. In other words, sheet conveyance force is set to be larger at the ends than in the middle to pull a sheet outward so as to prevent slack from being formed in the middle of the sheet, thus suppressing formation of the wrinkles.
Incidentally, it is effective to use an elastic sponge rubber layer formed by foaming silicone rubber so that a pressure roller has a heat insulating effect, or to use a void-containing elastic layer having an enhanced heat insulating effect through dispersion of a hollow filler (microballoons) in a silicone rubber layer so that a cured product contains gaseous portions.
However, when a roller main body of the pressure roller using the void-containing elastic layer has a reverse camber longitudinal shape, there is a problem in that a difference in sheet conveyance speed between a middle portion and end portions in the longitudinal direction of the roller main body does not occur, thus hindering stable control of sheet conveyance to cause formation of wrinkles. In other words, in a void-containing rubber elastic material such as sponge rubber, rubber itself has a large elasticity, and hence a difference in sheet conveyance force between the middle portion and the end portions is absorbed due to deformation of the rubber so that the void-containing rubber elastic material hardly affects behavior of a sheet.